1. Field of the Invention
The present invention relates to a sensor element of a gas sensor for determining gas components in gas mixtures, to a method for operating the same.
2. Description of Related Art
Gas-sensitive semiconductor-based field effect transistors are used to detect substances contained in a fluid stream, in particular gases in a gas stream. In general, impingement of the substance to be detected, for example a gas or a liquid or a gas or liquid mixture, results in a change in channel impedance and thus in a change in the current (called the “channel current”) flowing through the field effect transistor from the source electrode to the drain electrode. If semiconductor materials having a large band gap, i.e. a band gap of more than 3 eV, are used, for example such as gallium nitride or silicon carbide, this makes it possible, in principle, to use the gas-sensitive field effect transistors for sensor applications at temperatures up to 800° C.
At the selected working point of the gas-sensitive field effect transistor, the channel current without impingement of the substance to be detected (corresponding what is called a “null signal” or “offset”) is higher, often by orders of magnitude of usually 103, than the change in channel current resulting from impingement of a substance to be detected. Because of the poor signal-to-offset ratio, this imposes difficult requirements in terms of current measurement. A further problem which results is that the offset can be influenced by external interference effects. External interference effects are produced, for example, by temperature changes or by sensor degradation, which are not based on the presence of substances to be detected. Because of the signal-to-offset ratio that exists, the change in channel current resulting from interference effects can be of the same order of magnitude as, or in the most unfavorable case even greater than, the change occurring due to the presence of the substance to be detected. Because interference effects cannot be entirely precluded, the associated error in the measured signal is large, and in the worst case prevents sufficiently accurate determination of the substance to be detected.
U.S. Pat. No. 6,883,364 discloses the use of, inter alia, field effect transistors as sensors in handheld devices for detecting gases. Gas-sensitive resistors (called “chemoresistors”) are usually used here. The sensors used are, however, not suitable for drift compensation and for compensating for the offset of the chemoresistors.
Also known, from U.S. Pat. No. 6,165,336, are gas sensors that are constructed, for example, on the basis of a semiconductor gas sensor and that have a porous ceramic layer as a gas-permeable protective element in order to prevent the access of harmful gases.